I'll throw something out from left field. The weathered pits on the sides remind me of the cast of a particular type of Carboniferous Era plant fossil, namely Stigmaria. And it does look like sandstone, which most fossil casts from that era will be formed in. Just a thought, not sure Carboniferous Era fossils are even known from whatever state you found that in, and maybe those irregular pits are not the bark pattern seen on such casts. That could be an example of such a cast in which the pattern on the outer surface is greatly weathered. I think many fossil collectors would go there with an interpretation, while artifact collectors will go to pestle in interpretation. As well, there are other Carboniferous plants with a pitted bark pattern. I really believe, FWIW, that this is the first thing a fossil collector would think of when seeing that sandstone cylinder. A fossil cast in the round.

Of course, it could also simply be pecking from the creation of the pestle, and maybe that's the best bet.

I'll throw something out from left field. The weathered pits on the sides remind me of the cast of a particular type of Carboniferous Era plant fossil, namely Stigmaria. And it does look like sandstone, which most fossil casts from that era will be formed in. Just a thought, not sure Carboniferous Era fossils are even known from whatever state you found that in, and maybe those irregular pits are not the bark pattern seen on such casts. That could be an example of such a cast in which the pattern on the outer surface is greatly weathered. I think many fossil collectors would go there with an interpretation, while artifact collectors will go to pestle in interpretation. As well, there are other Carboniferous plants with a pitted bark pattern. I really believe, FWIW, that this is the first thing a fossil collector would think of when seeing that sandstone cylinder. A fossil cast in the round.

Of course, it could also simply be pecking from the creation of the pestle, and maybe that's the best bet.